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  <div class="question_difficulty">
   难度：Easy
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   <h1 class="question_title">
    136. Single Number
   </h1>
   <p>
    Given a
    <strong>
     non-empty
    </strong>
    &nbsp;array of integers, every element appears
    <em>
     twice
    </em>
    except for one. Find that single one.
   </p>
   <p>
    <strong>
     Note:
    </strong>
   </p>
   <p>
    Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?
   </p>
   <p>
    <strong>
     Example 1:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> [2,2,1]
<strong>Output:</strong> 1
</pre>
   <p>
    <strong>
     Example 2:
    </strong>
   </p>
   <pre>
<strong>Input:</strong> [4,1,2,1,2]
<strong>Output:</strong> 4
</pre>
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   <h1 class="question_title">
    136. 只出现一次的数字
   </h1>
   <p>
    给定一个
    <strong>
     非空
    </strong>
    整数数组，除了某个元素只出现一次以外，其余每个元素均出现两次。找出那个只出现了一次的元素。
   </p>
   <p>
    <strong>
     说明：
    </strong>
   </p>
   <p>
    你的算法应该具有线性时间复杂度。 你可以不使用额外空间来实现吗？
   </p>
   <p>
    <strong>
     示例 1:
    </strong>
   </p>
   <pre><strong>输入:</strong> [2,2,1]
<strong>输出:</strong> 1
</pre>
   <p>
    <strong>
     示例&nbsp;2:
    </strong>
   </p>
   <pre><strong>输入:</strong> [4,1,2,1,2]
<strong>输出:</strong> 4</pre>
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